1. Field of the Invention
This invention generally relates to cross-flow filtration. Wastewaters containing dissolved and/or suspended solids may be detoxified, solid and gaseous products may be separated in situ, oxides and salts may be removed from fluids, ion species may be separated, and salt solution feeds may be separated into a filtrate and a retentate by embodiments of the present invention.
2. Description of the Prior Art
Water exhibits dramatic changes in physico-chemical properties near its vapor-liquid critical point (374.degree. C. and 22.1 MPa). At the critical point, the density varies rapidly with small changes in temperature. This transition is shown by a fluctuation in density from 0.7 g/cm.sup.3 to 0.2 g/cm.sup.3 over the short temperature range of 374.degree. C. to 450.degree. C. At its critical point the density of water is 0.32 g/cm.sup.3, as compared with 1.0 g/cm.sup.3 at standard temperature and pressure conditions. Under supercritical conditions density and viscosity are directly related. At 500.degree. C. and 0.2 g/cm.sup.3 and at 500.degree. C. and 0.8 g/cm.sup.3, the viscosity is about 0.04 cP and 0.10 cP respectively. Under supercritical water conditions, the density and viscosity are lower than that of water at standard temperature and pressure conditions, so diffusivity and ion mobility are expected to be higher at supercritical conditions. These properties may be utilized for solid-fluid separation.
As shown in FIGS. 1A and 1B, one of the characteristics illustrating the structure of water in the supercritical region is the static dielectric constant. This parameter is related to hydrogen bonding and reflects the concentration of polar molecules in the water. The unusually high dielectric constant value of water under standard temperature and pressure conditions is due to the strong hydrogen bonding between water molecules. As the density of water decreases, hydrogen bonding also decreases. In addition, solvent polarity is reduced. The dielectric constant is about 78.5 at standard temperature and pressure conditions. At the critical point the dielectric constant is 5.
Dissociation of water in the supercritical region is a strong function of density and temperature, as shown in FIGS. 1A and 1B. Values for the ion product (pK.sub.w) first decrease from 14 at ambient conditions to 11 at 25 MPa and 250.degree. C. and then increase to 21.6 at 25 MPa and 450.degree. C. This means that fewer ions exist in supercritical water and that electrolyte association is favored over dissociation in the supercritical region.
Different solvation mechanisms are responsible for solubilities of salts and oxides in supercritical water. The solubility of the former is determined by ionization ability of water, while the solubility of the latter reflects the dissolution ability of water. Because of the decreased dielectric constant of supercritical water, water loses its capacity to mask charges of ions in solution, hence no extensive hydration shells are formed around inorganic electrolytes. As a result, inorganic electrolytes become insoluble under supercritical water oxidation conditions. However, the density and dielectric constant of supercritical water increase with increasing pressure. Therefore, both ionization and solubility of inorganic electrolytes are adjustable by temperature and pressure. The solubilities of selected inorganic salts and metal oxides in water are given in Table 1 for standard temperature and pressure conditions, and in Table 2 for supercritical water conditions.
TABLE 1 ______________________________________ Properties of Selected Salts and Oxides In Water at Standard Temperature and Pressure Melting Point Density Solubility Compound (.degree.C.) (g/cm.sup.3) (mg/L) (@.degree.C.) ______________________________________ NaCl 801 2.165 391,200 (100) NaHCO.sub.3 270 2.159 164,000 (60) Na.sub.2 CO.sub.3 851 2.533 455,000 (100) Na.sub.2 SO.sub.4 884 2.680 283,000 (100) Mg(OH).sub.2 350 2.360 40 (100) NaNO.sub.3 306.8 2.261 921,100 (25) 1,800,000 (100) NaOH 318.4 2.130 420,000 (0) KOH 360.4 2.044 1,070,000 (15) 1,780,000 (100) SiO.sub.2 (Quartz) 1610 2.660 Insoluble Al.sub.2 O.sub.3 2072 3.965 Insoluble .alpha.-Al.sub.2 O.sub.3 2015 3.970 0.98.sup.(1) .gamma.-Al.sub.2 O.sub.3 (1) 3.5-3.9 Insoluble ______________________________________ .sup.(1) transition to Al.sub.2 O.sub.3
TABLE 2 ______________________________________ Solubilities of Selected Salts and Oxides in Supercritical Water Temperature Solubility Compound Pressure (MPa) (.degree.C.) (mg/L) ______________________________________ NaCl 27.6 500 304 25.0 450 200 30.0 500 200 20.0 450 63 29.8 408 824 29.8 509 299 Na.sub.2 SO.sub.4 27.4 350 70,000 30.0 450 0.02 29.8 407 136 NaHCO.sub.3 29.8 509 86 CaCO.sub.3 24.0 440 0.02 Mg(OH).sub.2 24.0 440 0.02 NaNO.sub.3 27.6 450 991 27.6 475 630 27.6 500 540 SiO.sub.2 34.5 400 637 34.5 500 216 25.0 450 55 30.0 450 160 CuO 31.0 620 0.015 25.0 450 0.010 ______________________________________
The selection of an appropriate solids separation system must be based on the desired particle removal efficiencies and the physical process constraints. Particles to be removed in supercritical water oxidation of wastewater and sludges typically range between 0.1 and 10 microns. Microfiltration is capable of removing particles of this size.
FIG. 3 shows the operational principles for both cross-flow filtration and "dead-end" filtration. The advantage of the cross-flow filtration is that the feed suspension flows perpendicular to the filter surface at high shear rates to prevent solids build-up. The pressure drop that occurs over the filter element acts as the driving force for mass transfer. The pressure drop is a major factor to determine the maximum achievable flux rate across the membrane.
The filtration flux across the membrane can be reduced by clogging of the pores and the formation of a surface layer on top of the filter element. Clogging of the pores is caused by particles smaller than the pore size. The extent of clogging is dependent on pore shape as well as on particle size distribution and particle shape. The cake consists of a layer of particles with size equal or greater than the pore diameter. At some equilibrium thickness, the fluid shear force on the surface will equal the particle attraction to the membrane. At this point the layer reaches its maximum thickness. The fluid shear force increases with increasing fluid velocity, as a result, the thickness of the surface layer decreases and the flux over the filter element increases.
U.S. Pat. No. 4,378,976 describes an apparatus for removing solid particles and/or liquid droplets from a gas stream comprising a heating means in combination with a sonic agglomerator and a porous cross flow filter. U.S. Pat. No. 4,822,497 describes an aqueous-phase oxidizer and solids separator reactor, in particular, a two zone pressure vessel in which precipitates and other solids fall or are sprayed from a supercritical temperature super zone into a lower temperature sub zone.
A solution to the solids problem provided by an aspect of the present invention is to remove precipitates within the supercritical region using cross-flow filtration.